Signal to noise ratio indicator



Aug 20, 1963' w. L. GLOMB ETAL. 3,101,446

SIGNAL To NoIsE RATIO INDICATOR 5 Sheets-Sheet 1 Filed Sept. 2. 1960 SV P x Aug. 20, 1963 w. L. GLOMB ETAL 3,101,446

SIGNAL To NOISE RATIO INDICATOR Filed Sept. 2, 1960 3 Sheets-Sheet 2 WALrER L, cua/08 Y DONALO R. CAMPBEL B .fof/,v H. KRESSMAA/ WOM AGENT Al1g 20, 1963 w. L. GLoMB ETAL 3,101,446

SIGNAL To NoIsE RATIO INDICATOR 5 Sheets-Sheet 5 Filed Sept. 2, 1960 3,101,446 SEGNAL T NGISE RATIO TNDICATQR Walter L. Glomb, Clifton, and Donald R. Campbell, Nutley, NJ., and .lohn H.' Kressman, Easton, Pa., assignors to International Telephone and Telegraph Corporation, Nutley, NJ., a corporation of Maryland Filed Sept. 2, 1960, Ser. No. 53,764

2G Claims. (Cl. S25- 67) This invention relates to signal monitoring systems and more particularly to a'signal-to-noise ratio indicator.

-In the operation Iof electromagnetic Wave propagation sys-tems, such as communication, navigation and surveillance systems, it often becomes desirable to continuously monitor the system signal-to-noise ratio, particularlyf'in the neighborhood of threshold. A signal which is 1n-` dicative of Ithe signal-to-noise ratio is useful in the determination of system reliability in early phases of operation Iand can be utilized in subsequent operations as a squellch activating signal. In conjunction with some diversity receiving schemes, the signal indicating the ysignal-tonoise ratio can yalso be used to disable a particular receiver when operation below threshold is imminent.

An object of this invention is to provide an improved signal-tonoise ratio indicator.

Another object lof this invention is to provide a circuit producing an Ioutput signal indicating the signal-to-noise ratio which may be utilized for monitoring, squelch actlvating, or :diversity receivingpurposes.

A feature of this invention is the provision yof a circuit operable upon the composite sum of a sine wave plus noise to indicate the signalsto-noise ratio of a signal.

Another feature fof lthis invention is a circuit including means responsive to composite sum of a low frequency `sine wave disposed below the baseband signal and the thigh frequency noise disposed `above thebase band signal to produce a compressed sine wave and means coupled to the responsive means Ito detect the amplitude of the compressed 4sine Wave, the detected amplitude being the signal indicative' of the signabto-noise ratio `of a signal.

The above-mentioned yand other features and objects off this invention will become more apparent by reference to the following description Italien in conjunction with the accompanying drawings, in which: v

FIG. l is :a schematic diagram in block form of a signal-to-noise ratio indicator in accordance with the principles of this invention;

FIG. 2 is a graphical illustration useful in explaining the operation of the signalto-noise ratio indicator of FIG. l; y

FIG. 3 is a graphical illustration relating sine wave output and lreceiver input carrierato-noise ratio for several values of bias applied to the clipper circuit of FIG. l;

FIG. 4 is la schematic diagram in block forrn of a communication system incorporating the signfal-to-noise natio indicator of thus invention for evaluation of a multi-hop or multi-link communication system.l

Referring to FIG.` l, there is illustrated therein a communication system employing the signal-to-noise ratio indicator of this invention. The communication system is illustrated as including a terminal 1 for transmitting the baseband .signal from source 2 by means of transmitter 3 and antenna 4 to a receiving terminal4 5. Terminal 5 includes an antenna 6, ratio frequency (RF) amplifier 7, fa heterodyning arrangement including mixer 8 and oscillator 9, 'an intermediate frequency (IF) amplifier 10,

a limiter discriminator 11, a baseband ampliier 12 andy a baseband utilization device 13'. Although, in FIG. l,l

the communication system is illustrated to lbe a radio transmission system, `other transmission media, such asav atentQwl Y 2 open wire or coaxial cable media may be substituted 'al radio transmission medium. y

The signal-to-noise ratio 'of the communication system `outlined hereinabove is obtainedfby employing at terminal 1 a sine Wave source tov inject a sine 'Wave'having for.

a frequency fo into theinputcftransmitter 3I for'propav gation to terminal 5 along Withb aseband source. The frequency of the .sine wave is chosen to tbe below the band occupied by the baseband signal tand, hence, kis a low frequency sine w'ave. l propagation media between `antenna 4 Iand 6, the sine Wave and the baseband signal are propagated .through the saune medium between antennas 4 and 6 inthe presence of noise. Thus, if the -signalto-noise ratio of the sine Wave is detected, the results be indicative `of the signalto-noise ratio of the received; carrier signal. Therefore,

in yaccordance .the principles of this invention, the

' signal-to-noise A,ratio vindicator iscoupled to the output tof limi-ter discriminator llby means of switch 15 1'11 .the position illustrated. The output lof limiter discrimi` nator 11 is passed through a band reject lter 16 which removesthe lbasehandsignal `and leaves only the low frequency sine Wave and the out `of band or high frequency noise as illustrated in the ouwe 17. The sine wave and the out of band noise are applied to ampliiie'r 13 which *has la dynamic range adequate Ito pass these signals without clipping.` The output signal of arnpliii'el i '18 is applied to `a symmetrical clipper 19l including diodes Ztl and 21 and `a bias source 22. The amplitude of the bias voltage of source 22 is selected to be at some particular clipping level usually yexceeding thepeak amplitude of the sine WaveA alone. Following the clipping, the clipped signal is amplified by amplifier 235` and coupled to band pass filter 24 to extract :fromthe clipped signal the resultant sine wave only, Whichis compressed in yamplitude if noise is present prior to clipping. The output of band pass filter 24 is coupled .to detector 25 to ydetect the amplitude tot the resultant sine wave. The 'output of detector 25 is coupled to a utilizationdevice 26, such as 'a meter or other indicator, to measure the amplitude of the detected output. The measured output will Vbe an indication of the monitored signal-to-noi-se and may be translated to an actual ysignal-to-noise ratio by reference to prepared graphs fas hereinafter explained.

Thus, the signal-to-noise ratio indicator of this inven- .tion #operates on a combination of la low frequency sine |Wave tand out of band noise. It is sensitive to variations in propagation characteristics fat the extremities of the -trailic band, or in "other Words the baseband, due to the sine Wave being below the band `occupi-ed by the baseband and the out of hand noise being above the band occupied by the baseband. 'llhis choice fof :operating frequencies provides a measure of protection against frequency selective fading which is characteristic of long range overy the horizon propagation. The proposed method essentially Yoperates lon the principle that a sine Wave inthe presence of noise when clipped at some point,"` preferably ycorrespond to a received carriertonoise ratio values in l the'range from approximately 0 db (decibels) to 20 db Y or more depending upon the but of band noise available.4 v f Referring to FIG.-2, the operation of the signal-to-noise ratio lindicator of thisinvention is diagrammatically illustrated. Prior to clipping the sine wave, illustrated by the ,sine wave curve including they dotted portion 27, has

superimposed thereon of noise illustrated by curve 28.

Thus, at the input of clipper 19' there is present the coml aiutate Patented Aug. 20, 1963` In its propaga-tion through theV the sine g'wave'within the composite v vlave'ormand,l hence, alcompressed sine wave as illustrated by curve 29'. Filter 24 having a narrow' bandpass response'then extracts from the clipped composite waveform the resultantcompressed sine wave nly. The Clipping levels are illustrated by l-iriestlarid 31. Line 30 represents the negative bias voltage; offsource v2li ,with the diodes 20and 2,1 of clipperl 1 9 poled asindoated `and line` 3 1 represents the reference or -groundpoential -applied'to the 'cathodeof kdiode 2l". y

Under the. preferred arrangement, with switch lSinf the will extend fromfjust above the` highest traiiic vfrequency to Af'XZ' where' Afis the receiverv intermediate requency i i wave and the clipper bias is setto kequal ythe peak of thesine wave For examplefor a system having a 30` kc.'

. (kilocycles) I'F bandwidth vanda Since'wave at 8 kc. with deviation of i125 kc.fpeak,.assuming a post detection noise bandwidth of 50 kc. to 150 kc., then the maximum carrier to vnoise ratio is calculatedat +15 db.

lf additional sensitivity is require-dla reduced amplitude sine Wave can be used and the biascorrespondingly reduced. The n'raxirnumy range of operation of the indicator circuitfis .obtained when the lamplituderof the sine Wave is as small as possi-ble.

The description hereinabovehasbeen concerned with Y the evaluation of the signal-to-noiseratio or received car-` i rier to noise 4ratio on a single-link lof'ta communication positionv illustrated in V1,'the signilicant noiseband system as depicted in -FiG. 1. A 'Ilresignal-toaneise ratio indicator of this invention maybe'utilized 'to-.evaluate at ra giventerminal apluralityof communication hops coririected ina tandem Larrangement'such as indicated in v PEG; 4. kReferring to FIG. 4, there is illustrated therein bandwidth'. In lrnost practical frequencydivision modula.- tion systems, Af/ 2 will be severaltimes ythehighest traiiic v.channel frequency. It, however, the output is taken trom n, the 'output'of ibiaseb-andamplierlz by positioning switch 15 in its -alternate position; sensitivity maybe sacriiiccdy since the baseband amplifier may'have a somewhat narpower bandwidth than the Af/Z bandwidth inthe preferred arrangement. Although the sensitivity is sacrificed,I this alternative :arrangement maybe utilized where greater y 'ampliiication is requiredprior to application to the signal- `tov-noise indicator of this invention. .f

:The operationof the signal ltonoise ratio indicator can be proven, by the more formalrigorous.evaluation vof the 'spectral energy distributionof a clipped Rayleigh distribution representing the sum of the isine wave plus noise, vor the more informal evaluation represented bya Gaussian .distribution With slowly varying meanA value clipped at a ygiven level,'to be only a function of the ratio ofthe clipper v v biasfthe magnitude of the outputvof source 22, and the r.m.s,. (root mean square) value of the noise. While in plurality lof hops including termin-ala 38, a terminal 391,

and a terminal 40" connectedA in a tandem arrangement; It is to -be understood that more' than the number of teri minals illustrated may be included .inv the communication a system and the `system evaluated as outlined hereinbelow for'the two hops illustrated inl FIG. 4. 1By inserting the sine Wave signal iat terminal Miron-u source 4l, it is plo-ss'ible to evaluate the signal-.to-noisieratio of either the hop between terminals 38 and 39 or the composite signal-tonoise ratio of the entire Isystem from termina-l 3S to terminal 40. i a

Y Consider first the.' employment of a single sine wave such as the outputolsource 41, to evaluate the entire, system.

`An approximate evaluationv of theV signal-to-noise vratio vis obtained by transmitting the sine Wave along with the theory' this is true, severalpractical oonsiderationslimit the operating'rangeof the device. ,In a pnacticral fre- "queney modulation system, the lower limit'to the operation of the circuit is established by thecompression of the Y "modulation in the presence of noise'that `takes place in the frequency modulation (FM). ldeinodulation process.

This curve is representedby Where v0 is the normal output, v is the compressed output,

' and N/ C is the noisepower to carrier, powerratio. This curve is plotted' `in FIG. 3 as curve32 and represents the compression which occurs in anfFM 'system in theabfsence of any subsequent clipping. In -willbe noted that this compression. reaches 50% at a carrierfto noise ratio'of -l. dbVA as 4is'indicated bythe interception of dotted lines' -33 and'34. This compression alone could be used Ito monitor system performance. However, it is apparent that for most applications, the range, ofjoperation is about l0 "dbzlower -incarrier to noise ratio .than the range of interest.

With the. introduction of clipping circuit 19, as described hereinabove withrespect to FIG.-`1j,`it is possible to simu-y latecompression cur-ves such as indicated by curves" 35,36

and" 37 Where curve '35 is for a clipping bias greater than TV1', curve 2561's for a'clipping bias equal to v1, and curve 37 is lfor a Vclipping bias less than v1, where v1 equals lthe 'peak amplitude of the sine Wave at the input of clipperl9.r f The upper' range 'of carrier .to noi-se ratio is limited bythe amount of noise available and they magnitude ofthe sine I. wave. Since the clipping level cannot be madeplesfs than theA peak Vsine wave amplitudel without resulting in compression in the absence of' noise,` then the maximum value v of carrier to noise ratio at which the rcircuit willY clause 50%1oompression is thatat which the' r.m.s. value of noise is' equal to 0.707 of the peak amplitude of the sine baseband 'signal fromterminal 38 to terminal 39* wherein they signal is.. transmittedY from transmitter 4Z' to yan antenna 43 and, hence, to antenna 44 `and receiver 45. Receiver 45 is illustrated as including an RF amplifier, mixer,

- IF ampliiier, (and limiter discriminatori.' The'output of the limiter discriminator is coupled to the signal-to-noise ratio indicator including band reject 'filter 16, amplifier-128, clipper circuit 19', amplifier 23, iand band pass iilter 24,

Vand detectorI 25 as ydescribed hereinabove Iwith respect to FIG. 1. `When the compressed sine wave signal'isdetected in ldetector 25', f the detected lamplitude yisy coupled to transmitter 46 of terminal 39 and coupled from an tennai47 to antennaAS of terminal 40. Terminal 40I includes a receiver 49, also including an RF amplifier, mixer,

' in subsequent hop.

vIF amplifier, and limiter discriminator.-. The sine Wave having a frequency fo would then be detected again in another indicator including lband rejection iilter 16 and so forth as was described inconnection with FIG. 1. The

fjresultant detected 'sine Wave is coupled to the utilization device'wlich could beagmeter to provide an approximate evaluation of the signal-to-noise ratio of the entire system from terminals Sisto 40. This arrangement cannot, however, give the identity of the fading hop and the results are no longer unique. This latter eiect results from the fact that the resulting compression is` non-linear and the compression o-f a signal in one hopjsubstantially eifects the critical carrier-to-noise ratio at which lcompression occurs ,v 'Ilhe following arrangement would be employed to evaluate the individual hops lof fthe communication system permitting the identication of Ithe'hop that is diading or approaching; threshold. As before, the output rfrom sine l wave source 411,V would 'bev transmitted Ito' terminal 39 `Wherein the signal-to-noise'ratio would be indicated by employing .thevsine wa'vc having la frequency fo andthe out of bandinoise as` describedl before in connection .il-"16u 1. Filter V50 rejectsl the sine Iwave having a requency f5 so that spacious `information is vnot transmitted-through *,terminal 39." The detected lamplitude of the compressed s'inewave would be trlansfmitted from terminal 39l to termi- .Thal -40and`this signal' would be detected in filter Siland 

1. A CIRCUIT TO INDICATE THE SIGNAL-TO-NOISE RATIO OF A SIGNAL IN THE PRESENCE OF NOISE, SAID SIGNAL INCLUDING A SINE WAVE, COMPRISING MEANS RESPONSIVE TO THE COMPOSITE SUM OF SAID SINE WAVE AND SAID NOISE TO PRODUCE A RESULTANT SIGNAL INCLUDING SAID SINE WAVE COMPRESSED IN AMPLITUDE PROPORTIONAL TO THE AMPLITUDE OF SAID NOISE, AND MEANS COUPLED TO SAID RESPONSIVE MEANS TO DETECT THE AMPLITUDE OF SAID COMPRESSED SINE WAVE. 